Taking the Mystery out of Tilt-Shift Lenses
February 27, 2012
Don't be intimidated by tilt-shift lenses, which offer a bewildering variety of lens movements: rise, fall, shift, tilt, and swing. How do you know which movement to use? Relax. It's not as hard as it looks. When you use a tilt-shift lens, you are generally trying to achieve one of two goals: keep parallel lines parallel, or control depth of field by controlling the plane of focus. Let's tackle those goals one at a time.
You've probably had the experience of standing inside a forest and pointing a wide-angle lens upward. All the trees suddenly seem to lean inward toward one another. Or imagine you're standing on the lip of the Black Canyon of the Gunnison, an incredible, vertical-walled slit in the Earth that at Chasm View is 1,800 feet deep and only 1,100 feet wide. You put on your wide-angle lens, point it down to take in the abyss at your feet, and suddenly the canyon walls lean outwards. Abruptly, the awesome and intimidating Black Canyon of the Gunnison has lost its sinister aura.
To keep parallel lines parallel, be they tree trunks or canyon walls, you must keep the back of the camera, and therefore the sensor, perpendicular to the ground. A bubble level that fits into the hot shoe is the easiest way to ensure this. Unlike conventional lenses, whose lens axis is always centered on the sensor, tilt-shift lenses allow you to raise and lower the lens in relation to the sensor, as well as shift the lens side-to-side. To look up, raise the lens, a movement called rise. To look down, lower the lens, a movement called fall. The movement called shift, in which you move the lens left or right, is rarely used, but could be helpful if you were trying to control the perspective of a fence or wall receding into the distance.
It's quite true you can use Free Transform in Photoshop to achieve a similar effect, but with two drawbacks. The first, minor, drawback is that Photoshop must invent pixels to fill in the newly added area, causing some slight loss of sharpness. The larger drawback is that you can't see what you're doing in the field. You have to compose generously and hope the final composition is pleasing after the transformation, which will, in effect, crop off a triangular area from both the left and right sides of the composition.
The second major use of a tilt-shift lens is control of depth of field by controlling the plane of focus. With a conventional lens, the plane of focus is always parallel to the plane of the sensor. You can focus on something nearby, or something far away, but you can't focus on both simultaneously. A tilt-shift lens allows you to tilt the lens in relation to the sensor. That, in turn, tilts the plane of focus so it is no longer parallel to the sensor plane. If you choose, you can make the floor on which you're standing be the plane of focus. Or, more usefully, you can position the plane of focus so it passes through the top of every flower in a meadow. Swing is the same movement optically as tilt, just with the lens mechanism rotated 90 degrees. Instead of the top of the lens tilting forward and down, the side of the lens swings to the right or left, pivoting about a vertical axis. You use swing to keep an entire wall (a vertical surface) in focus as you look along its length. You use tilt to keep an entire horizontal surface in focus. In effect, in the right situation, you can focus on near and far simultaneously and achieve incredible depth of field. With a 24mm tilt-shift lens, I can often achieve razor-sharp focus on flowers one foot from the camera as well as the most distant peaks. The best depth-of-field you can achieve with a conventional 24mm lens is about two feet to infinity, and that image will only appear razor-sharp throughout in an 11x14 print. There is no Photoshop equivalent to the optical effects of tilting the lens.
Tilting the lens won't help you achieve greater depth of field if the subjects at the top and bottom of the frame are at equal distances from the camera. The most common example of this situation is a shot taken inside a forest, where both the top and bottom of an individual tree trunk are equidistant from the camera if your angle of view is parallel to the ground ‒ you're looking horizontally, in other words, not up or down.
Deciding on the right amount of rise or fall is easy. Position the camera back perpendicular, look through the lens, and raise or lower the lens until you've achieved the desired composition. Choosing the right degree of tilt is a bit trickier. Here's my method.
I start by zeroing all the movements and composing my image. Then I focus manually on the most distant part of my subject. Next, I focus on the closest part of my subject, then focus so both the most distant and closest objects appear to be equally fuzzy. By focusing first far, then near, I can refresh my memory on what truly sharp looks like, which makes it easier for me to judge when both parts of the subject are equally fuzzy. Then I slowly tilt the lens until both foreground and background appear sharp. If there is no such point, I re-zero the movements and start over, refocusing the lens so both foreground and background are equally soft, then tilting to bring both of them into focus. Remember that perfection is impossible. Unless you're photographing some ornate, tiled floor, it's unlikely every part of your subject will fall into one plane. When you've got the focus and tilt as close as possible, stop down to f/11 or f/16 to bring any remaining parts of the subject into the depth of field.
Some people like to set the correct amount of tilt by turning on Live View, magnifying the image to 5x, then scrolling back and forth from the top to the bottom of the frame as they adjust the tilt mechanism. This certainly works, but it's much slower than my method. I do use Live View occasionally to double-check my settings.
Both Canon and Nikon make tilt-shift lenses. The most useful focal length for a landscape photographer, in my opinion, is the 24mm tilt-shift. With lenses from both manufacturers, you cannot use any automatic exposure mode when using lens movements. You must use manual exposure. If you want to use the camera's meter to calculate the exposure, you must do so with all movements zeroed, then set the desired movements. Something about lens movements disrupts the normal operation of the in-camera meter. You can also use a hand-held meter to calculate the correct exposure, then set the exposure on the camera using manual exposure mode even if you've already used movements. In that case, however, the in-camera meter will tell you that you've set the wrong exposure.
Tilt-shift lenses aren't for everyone. They are certainly bulky, heavy, and expensive. If you are fanatical about the highest possible quality, however, you may find them invaluable. I certainly do.
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